Double-wall plastic tubing in which the outer wall has transverse corrugations and the inner wall is smooth

ABSTRACT

In the manufacture of double-wall plastic tubing in which the outer wall has transverse corrugations and the inner wall is smooth, in the case of annular transverse corrugations the air pressure between the two walls cannot be balanced with the external air pressure, resulting in deformations of the inner tube. To permit an equilibrium of pressure before the inner wall hardens, openings are pierced in the outer wall or openings are produced between the two walls for communication between the inner chambers of the transverse corrugations. After the hardening of the inner tube, these openings are reclosed by fusion.

This is a continuation of application Ser. No. 967,807, filed Dec. 8,1978, now abandoned, which is a division of application Ser. No.823,778, filed Aug. 11, 1977, now U.S. Pat. No. 4,145,387, issued Mar.20, 1979.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to a method of making double-walled plastictubing in which the outer wall has transverse corrugations and the innerwall is smooth, the transverse corrugations being produced by means ofrecirculating molds.

DISCUSSION OF PRIOR ART

It is known to produce such tubing by extruding two tubes fromconcentrically disposed annular dies, the outer tube being carriedbetween two oppositely circulating sets of half molds which jointogether along a path to form the whole molds and which are providedwith depressions and elevations corresponding to the transversecorrugations, and the heat-softened tube being made to conform to themolds by a pressure difference. The tube extruded from the inner annulardie is introduced into this outer tube provided with the said transversecorrugations, and is brought into engagement with the outer tube by apressure medium, so that the two tubes can be fused together.

In this method, to enable the inner tube to have a smooth surface andprevent it from bulging inwardly or outwardly, preheated air having aslight excess pressure is introduced into the space between the twotubes and into the interior of the inner tube. A pressure equilibriumthen establishes itself during the cooling of the outer tube. Thepractice of this known method presents no difficulty when the transversecorrugations of the outer tube are helical corrugations and thus form anuninterrupted passage whereby the air pressure within this passage canbalance out so that no overpressure or underpressure develops.

The situation changes when the transverse corrugations of the outer tubeare not helical but full circular corrugations. In this case anequilibrium of pressure is no longer possible after the inner tube hasbeen welded in place. Double-wall tubing with circular transversecorrugations, however, are preferred in all cases where special value isplaced on the avoidance of leaks in the tube couplings and theattachment of branches. In the case of tubes with helical corrugations,care must be taken at the tube ends and at branches to see that thepassage is carefully sealed between the two tubes. In tubes havingannular corrugations, this is not necessary because each of the annularpassages is sealed off, so to speak, from the adjacent passage.

The production of such tubes with annular corrugations presentsconsiderable difficulty, because the air pressure within the annularcorrugations changes during the cooling process. In the fabrication ofpolyolefin chlorides, especially polyvinyl chloride, these difficultiescan be overcome by introducing air at elevated temperature and elevatedpressure into the space between the two tubes. Upon cooling, the airpressure within the annular passages decreases, but the pressurereduction can be kept within such limits that, upon further cooling, thepressure difference will not be so great as to draw the inner tube intothe annular passages. In the fabrication of polyolefins, such aspolyethylene or polypropylene, or polyamides and other such highpolymers, substantially greater difficulties are involved. Polyolefins,however, have the advantage over the polyolefin chlorides that they areeasy to weld and have greater resistance to cold. Furthermore,polyolefins when burned do not pollute the environment, whereas theburning of polyvinyl chlorides produces hydrochloric acid. Thedifficulties are to be attributed above all to the fact that thefabricating temperatures of the polyolefins are considerably higher thanthose of the polyolefin chlorides. Accordingly, the volume reduction ofthe air entrapped in the circular channels is considerably greater thanin the case of the polyolefin chlorides and can no longer be compensatedby introducing air at higher pressure into the space between the twotubes. In addition, the polyolefins have a low thermal conductivity. Ifthe outer, corrugated tube is cooled, the heat can escape outwardly fromthe inner tube only through the weld seams, i.e., the inner tube remainsplastically deformable for a long time after the solidification of theouter tube.

THE INVENTION

All these difficulties are overcome in tubes whose outer wall hasannular transverse corrugations by producing in the outer wall orbetween the two walls openings which connect the interiors of thetransverse corrugations to one another or to the exterior and which areclosed by fusion after the inner tube has cooled.

If openings are produced whereby the adjacent annular corrugationscommunicate with one another, the same situation exists as in the caseof tubes having helical transverse corrugations. However, to avoid thedisadvantages of the latter tubes, the communication openings mustafterwards be closed, and this presents difficulties if the adjacenttransverse corrugations are connected to one another by the openings.

According to the further invention, therefore, the openings are piercedfrom the outside. In this manner, each individual annular chamber is incommunication with the outside atmosphere and, when the air within theannular chamber cools, air can enter from the outside, so that adepression is avoided. To be able to reclose the pierced openings afterthe cooling of the tube, it is desirable, by means of depressions in themolds, to form corresponding, nipple-like projections on the annularribs, into which the openings are pierced. By means of fusion with a hottool or ultrasonic fusion, the openings in the projections can beclosed, the projections being pressed flat and thereby made to conformto the rest of the profile.

The piercing of the openings is performed as the tube emerges from thetrain of molds. The openings, however, can also be pierced within themold train. For this purpose, bores can be provided in the molds whosesize matches the thickness of the needles for the piercing of theopenings. During the formation of the tube, these needles are in aposition wherein their points still do not extend into the cavity of themolds. As soon as the outside wall is sufficiently stable in shape,preferably shortly before the mold train opens, the needles are thenpushed through outside guiding members into the outer wall andwithdrawn.

When the tube leaves the mold train and while it is being furthercooled, a depression establishes itself within the annular corrugations,so that air flows into the annular corrugations through the piercedopenings. Not only air is thus admitted into the corrugation cavity, butalso high atmospheric moisture and also, under certain circumstances,water which is used for the cooling of the outer tube. If this mixtureof air and water strikes the outside of the inner tube, the latter isdeformed by evaporation and undesirable inward bulges are formed in theinside wall, which not only look unattractive but also reduce theusefulness of the tubing because they increase its resistance to flow.It has been found that such damage can be prevented by piercing theopenings into the wall portions which are vertical or nearly vertical inthe forming of the tube, i.e., they are pierced from the side, so thatwhen the air enters, any water droplets it may entrain will not impingeupon the inside wall opposite the opening, but will drop downwardly intothe annular chamber where they will spread out and evaporate.

This invention is also directed to a double-walled plastic tube made bythe method of the invention, whose outer wall has annular transversecorrugations and whose inner wall is smooth, and in which the outer tubewall has nipple-like projections which have been pierced and the holeshave been reclosed by fusion. In such tubes the scars where such fusionwas performed can be seen, as is not the case in tubes which have notbeen made by the method of the invention. In particular, the inventionrelates to a double-walled plastic tube of polyolefins or polyamideshaving a coarse profile, i.e., tubes in which the height and width ofthe corrugations amount to, say, 8 to 12% of the inside diameter; forexample, in a tube of an inside diameter of 100 mm, the outside diameterof the corrugations amounts to 120 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments which explain the invention are represented to FIGS. 1 to 9.FIGS. 10 to 14 illustrate the process and apparatus employed therefor.

FIG. 1 is a cross-sectional view taken through a portion of a tube withan opening made by piercing;

FIG. 2 is a horizontal cross section taken through the same tube;

FIG. 3 shows the same longitudinal section with the openings fused shut;

FIG. 4 shows a known double-wall tube to explain the deformation due topressure;

FIG. 5 shows a cross-section taken through a portion of a tube with anopening made by piercing;

FIG. 6 shows a vertical longitudinal section taken through the tube ofFIG. 5;

FIG. 7 shows a cross section taken through a portion of a tube withconnecting openings between adjacent circular corrugations;

FIG. 8 shows a partial longitudinal section taken through the tube ofFIG. 7;

FIG. 9 shows a partial longitudinal section taken through a tube havingprojections formed laterally on the corrugations;

FIG. 10 shows schematically a complete production installation for theproduction of double-wall tubes;

FIG. 11 shows a sector of this installation;

FIG. 12 shows a cut according to line XII--XII through the welder;

FIGS. 13 and 14 show various embodiments of molds.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The tube of FIGS. 1 to 3 consists of the smooth inner tube 1 and theouter tube 2 formed into annular transverse corrugations. Through anappropriate configuration of the molds, projections 3 are formed on theannular corrugations of the outer tube 3 at portions which standvertically upon the emergence of the tube from the old train, i.e., theannular cavity 5 between the inner tube 1 and the outer tube 2 isvertical at this point. Openings 4 are pierced in the projections 3,very small openings of a few tenths of a millimeter sufficing to assurepressure compensation. If droplets of water are entrained by the airentering through the openings 4, they will fall, as indicated by thearrows 6 in FIG. 1, into the annular cavity which is vertical orvirtually vertical at this point, where they will evaporate withoutcausing any damage.

The pressureconditions on a tube in which no pressure equalizationopenings are present are indicated in FIG. 4.

In the mold train the outer tube 12 is cooled to such an extent that itsshape is stable. Since the inner tube 11 can yield its heat only throughthe narrow heat bridges 13 at which the outer and inner tubes are weldedtogether, the inner tube has still not cooled sufficiently to have astable shape, but is still in a heat-softened state. To remove the heatfrom the inner tube as well, the outer tube must be intensively cooled;this can be done by carrying the tube through a water bath or sprayingit with water.

The air in the annular cavity 15, which had a temperature of over 200°C. corresponding to the softening temperature of the plastic when thetwo tubes were fused together, then cools to about room temperature.Even if supporting air at a pressure of 1.2 atmospheres has beenintroduced, a vacuum of about 0.4 atmosphere will result in annularcavity 15 after cooling down to room temperature, and this pressuredifference will force the still heat-softened inner tube, asrepresented, into the annular corrugations of the outer tube, so thatthe inside of the tube will not be smooth as desired, but will itselfhave annular ribs which very appreciably increase its resistance toflow.

In the subject of the invention, however, the pressure can reach anequilibrium with the external air pressure through the openings made bypiercing, i.e., as soon as the openings are pierced, no vacuum oroverpressure will be produced in the annular cavities, and the innerwall will be able to harden without any bulging, as represented in FIGS.2 and 3. In addition, any slight inward or outward bulging that may haveformed prior to the piercing of the openings due to the shrinkingtendency of the heat-softened plastic will also be compensated.

FIGS. 5 and 6 indicate the undesirable effect that can be produced ifthe openings are pierced vertically downward into the tube. The tuberepresented in FIGS. 5 and 6 again consists of an inner tube 21 and anouter tube 22 on which projections 23 are formed, into which thepressure equalizing openings 24 are pierced. If in this case waterdroplets are entrained by the inflowing air, they will fall, asindicated by the arrows 25, on the outer surface of the inner tube 21opposite the opening 24. Here the droplets of water will evaporate andproduce a strong cooling effect and hence a deformation of the innerwall, which then will have unattractive bulges 26 at these points, whichwill increase the resistance to flow.

Instead of piercing the openings from the outside, projections 33 can beproduced on the inner corrugations during the forming of the outertubes, as shown in FIGS. 7 and 8. If the inner tube 31 is then forcedagainst the corrugated outer tube 32, passages 35 will remain open atthese points between the individual annular chambers 34, so that theindividual annular chambers will be in communication with one another.Consequently, the pressure between the annular chambers can equalizeitself, so that in this case, too, no deformation of the inner walltakes place. Afterwards, after the outer and inner tube have hardened,the communicating passages 35 can be closed by pressing the projections33 flat with a tool heated to the softening temperature. This method hasthe advantage that no scars will be visible on the outer ribs. Theclosing of the passages 35, however, is not as easy to accomplish as theclosing of the air inlet openings 4 and 24.

To avoid scars on the outer circumference of the annular ribs, theprojections 43, as indicated in FIG. 9, can also be formed laterally onthe annular ribs. The pressure equalizing openings 44 can in this casebe pierced into the projections at an angle from above.

The production plant shown in FIG. 10 consists of a double spraying head50 which, for instance, can be an extruder head according to U.S. Pat.No. 3,994,644, the disclosure of which is hereby incorporatedspecifically herein by reference, of a forming machine 51, which can bean apparatus according to U.S. Pat. No. 3,776,679, whose disclosure isalso specifically incorporated herein by reference, or any other knownforming machine for the production of plastic tubes with transversecorrugations, the device 52 for the piercing of the holes into the outertube, a cooling system 53, the welding device 54 lfor the welding of theopenings produced by the device 52 and a separating device 55.

The shaping machine 51 contains, as shown schematically, two mold trainswith molds conducted in the circular course, which produce thetransverse corrugations of the outer tube. FIGS. 13 and 14 show crosssectional cuts through molds of the mold train for the production of atube where, according to FIGS. 7 and 8, the hollow spaces of adjacentribs are in connection with one another through channels 35. For thispurpose, as shown in FIG. 13, the projecting ribs have openings 56 inthe mold. FIG. 14 shows another embodiment of the molds wherein on theone side only every second rib has an opening 57, whereas in the othermold half the intermediate ribs have openings 58. In both cases it isattained that at the place of the hole the outer tube is not pressedonto the inner tube so that the adjacent cavities remain connectedthrough a passage 35 (FIG. 7). After the cooling of the tubes thesechannels can be heated by a simple device and pressed together.

The tube 59 consisting of an outer tube provided with annular transversecorrugations and an inner, smooth tube leaves the shaping machine 51 ina condition where the outer tube is sufficiently cooled at least on itsouter side that it can no longer be plastically deformed, whereas theinner tube still has a substantially higher temperature. Since by thehigher temperature of the inner tube the outer tube would again beheated to softening, the tube 59 must be led into the cooling device 53as quickly as possible. Between the shaping machine 51 and the coolingdevice 53 there is the perforator 52, whose effective portion is shownenlarged in FIG. 11. This device contains a cogwheel 60 which is drivensynchronously with the shaping machine 51 so that its teeth interlockinto the corrugations of the tube 59. In this cogwheel there areplungers 61 radially movable which are pressed outwardly by the springs62. On each plunger there is mounted a pin 63. During the rotation ofthe cogwheel in the direction of the arrow 64, the plungers are pressedinwardly by a tension ridge 65. At the end of the tension ridge 65 whichis closely in front of the vertex of the cogwheel 60, the plungers areset free and move rapidly forward by the effect of the tight springs 62,an opening being pierced in each case by the pin 63 into the ribs of thetube 53. During the subsequent movement of the tube and the cogwheel thepins are again extracted from the tube. Through this opening, the innerpressure within the hollow spaces of the ribs between the outer and theinner tube can adjust to the atmospheric pressure. Preferably, theapparatus 52 is not arranged below the tube 59 as shown, but at the sideof the tube staggered by 90° against the drawing plane so that theopenings are not pierced from below but from the side, as explainedabove with reference to FIG. 1.

Adjacent to the device 52, the tube enters the cooling device 43 whereit is sprayed with cold water from all sides by means of the nozzles 67.This cooling device is depicted in abridged form in FIG. 10. In case oftubes with larger dimensions, there are required cooling devices whichare three to five times as long as the forming machine 51. In thecooling device there are arranged conveyor belts 68 which are alsoactuated synchronously with the device 51 and do not only have thepurpose of moving the tube 59 through the cooling device, but alsoinsure that during the cooling the tube is not submitted to form changesthrough undesired shrinkage of the cooling plastic.

The tube 49 leaves the cooling device 53 in a condition in which theinner tube is also sufficiently cooled so that form changes no longerhave to be suspected. Now the tube 59 reaches the welding device 54 bywhich the holes in the warts are welded closed. This welding devicemust, of course, be in the same position as the perforator 52. Thiswelding device contains a welding prism 70 (FIG. 12) which is heated tothe required temperature by means of a heating device 71 and which isbeveled onto the tube 59 too strongly so that the welding heat only actson the tube within the region of the warts 4 or 24. By means of thiswelding device, the opening pierced by the device 52 is again closed.The welding prism 70 lies longitudinally movable on an inclined plane 72(FIG. 10) and can be moved on same by means of the spindle 73 in orderto allow the exact adjustment of the distance of the welding prism 70from the tube 59. In order to protect the tube 59 beyond the region ofthe warts from the heat radiation of the welding prism 70, the same issurrounded by a water-cooled cover 74. After the openings in the wartsare welded, the tube is finished and can be cut off in the device 55 inthe lengths desired in each case.

The cutting device 55 moves on a path 77 and is taken along by the tubeduring the cutting of the tube by gripping devices 78 so that thesectional planes are exactly at right angle to the tube axis.Preferably, the welding device 54 and the cutting device 55 are arrangedon a carriage 80 which can be moved by a hydraulic cylinder 81. Thesynchronous running of the perforator 52 and the conveyor belts 68 withthe forming machine 51 is insured by the common main shaft 82.

The openings formed in the walls of the annular transverse corrugationsor between adjacent corrugations generally have as the largest dimensiona dimension in the range between 0.8 and 2.0 mm. Generally speaking, theopenings are circular in cross section but can, of course, be oval,rectangular, square, triangular or any other shape.

What is claimed is:
 1. A double-wall plastic tube whose outer tube hasannular transverse corrugations and whose inner tube is smooth, saidouter tube having wart-like elevations having an opening therethrough,said outer tube and said inner tube being fused together, prepared by aprocess comprising extruding two tubes from two concentrically disposedannular dies, the outer tube being carried between two oppositelycirculating sets of mold halves which join together along their path todefine whole molds, the molds being provided with annular depressionsand elevations and the outer plastic tube being made to conform to themolds by a pressure differential whereby it obtains annular transversecorrugations and an inner tube is extruded from the inner annular die toengage and fuse to the outer tube by a pressure medium, said openingbeing formed in said outer tube, or between said inner tube and saidouter tube before said inner tube has hardened, said openingcommunicating with the region defined between the wall of said annularcorrugation and said inner tube with the next adjacent region defined bythe wall of a next adjacent corrugation and said inner tube or with theexterior beyond said outer tube and thereafter cooling the double-walledtube to harden said inner tube.
 2. A double-wall plastic tube accordingto claim 1 wherein said inner tube is a polyolefin or polyamide.
 3. Adouble-wall plastic tube according to claim 2 wherein said inner tube ispolyethylene or polypropylene.
 4. A double-wall plastic tube accordingto claim 1 wherein the height and the wave length of the transversecorrugations of the outer wall amount to about 8 to 12% of the insidediameter.
 5. A double-wall plastic tube whose outer tube has annulartransverse corrugations and whose inner tube is smooth, said outer tubehaving sears on the outer circumference of the annular corrugations,prepared by a process wherein two tubes are extruded from twoconcentrically disposed annular dies, the outer tube being carriedbetween two oppositely circulating sets of mold halves which jointogether along their path to define whole molds, the molds beingprovided with annular depressions and elevations and the outer plastictube being made to conform to the molds by a pressure differentialwhereby it obtains annular transverse corrugations and an inner tube isextruded from the inner annular die to engage and fuse to the outer tubeby a pressure medium, an opening being formed in said outer tube orbetween said inner tube and said outer tube before said inner tube hashardened, said opening communicating with the region defined between thewall of said annular corrugation and said inner tube with the nextadjacent region defined by the wall of a next adjacent corrugation andsaid inner tube or with the exterior beyond said outer tube, thereaftercooling the double-wall tube to harden said inner tube and thereafterreclosing the opening by applying sufficient energy to fuse the sameclosed whereby to form said sears.